From one girl to another... ;)
I have not personally tried this spell, but I can tell you that a spell isn't necessary to help your breasts develope.
As most women tend to have insecurities about their bodies, or simply wish to improve something for their own preference, there have been many simple ways discovered to help your body grow in a healthy, desirable manner.
Often times what you consume/eat is a good way to obtain desired effects. Breasts are more easily developed by consuming certain proteins like that found in some produce, and meats. Do a bit of research to find the specifics, and then go shopping. ^_^
There are also exercises one can do that tightens the pectorials which give women that often desired firm and perky look.
Wear clothing with some protective reinforcement as not to damage the tissue within your chest too badly. Also, it helps to make sure your chest has room to grow. :P